This invention is directed to a method of clarifying Bayer process liquors using salicylic acid containing polymers.
The Bayer process is almost universally used for the production of alumina from bauxite ore. The process involves pulverizing a bauxite ore, slurring it in caustic soda solution and digesting it at elevated temperatures and pressures. The caustic soda solution dissolves oxides of aluminum to form an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore (referred to as xe2x80x9cred mudxe2x80x9d) are then separated from the aqueous phase containing the dissolved sodium aluminate. This separation typically occurs through sedimentation, which is often aided by a flocculent, and filtration. Once separated, alumina trihydrate is precipitated from the aqueous sodium hydroxide and collected as product.
In more detail, the pulverized bauxite ore is fed to a slurry mixer where a caustic slurry is prepared. The slurry makeup caustic soda solution is typically spent liquor (described below) and additional caustic soda. The bauxite ore slurry is diluted and passed through a digester or a series of digesters where, under high pressure and temperature, about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. After digestion, the slurry passes through several flash tanks wherein the pressure of the digested slurry is reduced from several atmospheres to one atmosphere and the temperature of the slurry is reduced from about 200xc2x0C. to about 105xc2x0 C.
The aluminate slurry leaving the flashing operation contains about 1 to 20 weight percent solids, which solid consists of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solids may be removed from the aluminate liquor with xe2x80x9csand trapxe2x80x9d cyclones. The finer solids are generally separated from the liquor first by gravity settling aided by a flocculant and then filtration, if necessary. In some cases, the slurry of aluminate liquor leaving the flash tanks is diluted by a stream of recycled washer overflow liquor. Any Bayer process slurry taken from the digesters through a subsequent dilution of the slurry, including the flash tanks, but before the primary settler, is referred to hereinafter as the primary settler feed.
Normally, the primary settler feed is thereafter fed to the primary settler (or decanter) where it is treated with a flocculent. As the mud settles, the clarified sodium aluminate solution (referred to as xe2x80x9cgreenxe2x80x9d or xe2x80x9cpregnantxe2x80x9d liquor) overflows to a weir at the top of the vessel and is collected. This overflow from the primary settling tank is then passed to subsequent process steps.
The clarity of the primary settler overflow is crucial to efficient processing of alumina trihydrate. If the aluminate liquor overflowing the settler contains an unacceptable concentration of suspended solids (at times from about 10 to about 500 mg suspended solids per liter), it must be further clarified by filtration to give a filtrate with no more than 10 mg suspended solids per liter of liquor. The treatment of the liquor collected after the primary settlement to remove any residual suspended solids before alumina trihydrate is recovered is referred to as a secondary clarification stage.
The clarified sodium aluminate liquor is cooled and seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH)3. The alumina trihydrate particles or crystals are then classified by particle size and separated from the concentrated caustic liquor. A flocculant is used to aid in this classification and separation process. The very fine particles of alumina trihydrate are returned as the seed crystals and the coarser particles are collected as product. The remaining liquid phase, referred to as xe2x80x9cspent liquor,xe2x80x9d is then returned to the initial bauxite slurry make up and digestion step and employed as a digestant after reconstitution with caustic.
The settled solids of the primary settler are withdrawn from the bottom of the settler or decanter (and referred to as xe2x80x9cunderflowxe2x80x9d) and then passed through a countercurrent washing circuit for the recovery of sodium aluminate and soda. Overflow liquor from the first washing vessel (or xe2x80x9cthickenerxe2x80x9d) is recycled either as primary settler feed, diluting the slurry as it leaves the flash tanks, and/or it may be passed to filtration along with the overflow from the primary settler.
The partial separation of the red mud solids from the pregnant liquor in the primary settler (or decanter) is expedited by the use of a flocculent. This initial clarification of the pregnant liquor is referred to as the primary settler stage. Flocculating agents, such as liquid emulsion polymers, dry polymers and polysaccharides including starch, are commonly used to improve the separation of the insoluble red mud solids by increasing the rate at which these solids settle, by reducing the amount of residual solids suspended in the liquor, and by decreasing the amount of liquor in the settled solids phase, or underflow. Flocculation performance is critically important in the primary settling stages. Red mud solids comprised mostly of iron oxides (typically at least about 50 weight percent of the red mud solids), together with silicon oxides, calcium titanates, calcium phosphate, aluminum hydroxide, sodium alumino-silicates and other materials, commonly represent from about 5 to about 50 weight percent of the materials of the bauxite ore. Generally, these red muds are composed of very fine particles, which hinder the desired rapid and clean separation of the red mud particles from the solubilized alumina liquor. Improving the rate of separation improves the overall process efficiency and increases the output of alumina production. Improving the clarification of the process liquors reduces the need for filtration and further purification and can also increase alumina production. If the separation of the red mud particles is not clean, the resultant solubilized aluminate liquor will require a more extensive treatment to remove residual solids, and/or the alumina trihydrate recovered will contain levels of impurities that are undesirably high for many end-uses of the alumina.
In its principal aspect, this invention is directed to a method of flocculating suspended solids in a Bayer process liquor comprising adding to the liquor an effective amount of a polymer comprising salicylic acid groups.
The polymers of this invention effectively flocculate suspended solids in Bayer process liquors. In particular, use of these polymers in Bayer process caustic aluminate streams reduces the suspended red mud solids and significantly reduces the need for filtration of the pregnant liquor. Lower solids in the overflow liquor also reduce the amount of impurities such as iron oxide and other minerals, thus improving the purity of the alumina produced during precipitation.
The polymers of this invention also effectively clarify alumina trihydrate from Bayer process streams. During continuous or batch precipitation of the alumina trihydrate, coarse particles are separated from fine crystals primarily by gravity settling. A slurry of fine particles is sent to a series of secondary and tertiary clarifiers to concentrate the particles according to size. Flocculation and settling of the very fine particles is significantly improved by the addition of the polymers of this invention, resulting in reduced aluminum tri-hydrate solids in the spent liquor when compared with conventional processes, including the use of polysaccharides such as starch and dextran and/or combinations with polymers of acrylic acid and salts thereof.
The polymers of this invention show excellent affinity towards alumina trihydrate particles, flocculating such particles, and increasing the rate at which these particles settle. The very fine particles of alumina trihydrate can then be returned as seed crystals in the primary crystallization step. Use of the polymers of this invention reduces the suspended fine aluminum tri-hydrate in the tertiary classifier overflow, thereby improving the recovery of the alumina, ensuring that less alumina is recycled to digestion with the spent liquor.
Definitions of Terms
xe2x80x9cAcylxe2x80x9d means a group of formula xe2x80x94C(O)R where R is alkyl or aryl. A preferred acyl is acetyl (Rxe2x95x90CH3).
xe2x80x9cAIBNxe2x80x9d means 2,2xe2x80x2-azobis(2-methylpropionitrile), available from E. I. DuPont de Nemours and Co., Wilmington, Del. under the tradename Vazo(copyright) 64.
xe2x80x9cAIVNxe2x80x9d means 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile), available from E. I. DuPont de Nemours and Co., Wilmington, Del. under the tradename Vazo(copyright) 52.
xe2x80x9cAlkylxe2x80x9d means a monovalent group derived from a straight or branched chain saturated C1-C4 hydrocarbon by the removal of a single hydrogen atom. Representative alkyl groups include methyl, ethyl, n- and iso-propyl, n-, sec-, iso- and tert-butyl, and the like. A preferred alkyl is methyl.
xe2x80x9cAlkyl (meth)acrylatexe2x80x9d means the alkyl ester of acrylic acid or methacrylic acid.
xe2x80x9cAnionic monomerxe2x80x9d means a monomer as defined herein which possesses a net negative charge above a certain pH value. Representative anionic monomers include base addition salts of acrylic acid, methacrylic acid, itaconic acid, 2-acrylamido-2-methyl propane sulfonate, sulfopropyl acrylate or methacrylate or other water-soluble forms of these or other polymerizable carboxylic or sulfonic acids, sulphomethylated acrylamide, allyl sulphonate, sodium vinyl sulphonate, and the like.
xe2x80x9cArylxe2x80x9d means an aromatic monocyclic or multicyclic ring system of about 6 to about 14 carbon atoms. Representative aryl groups include phenyl, naphthyl and anthracenyl. A preferred aryl is phenyl.
xe2x80x9cBase addition saltxe2x80x9d means the salt resulting from reaction of a carboyxlic acid (xe2x80x94CO2H) group with a suitable base such as the hydroxide, carbonate, or bicarbonate of a metal cation or tetraalkylammonium cation, or with ammonia, or an organic primary, secondary, or tertiary amine of sufficient basicity to form a salt with the carboxylic acid group. Representative alkali or alkaline earth metal salts include sodium, lithium, potassium, calcium, magnesium, and the like. Representative organic amines useful for the formation of base addition salts include, ethylamine, diethylamine, ethylenediamine, ethanolamine, diethanolamine, piperazine, and the like. Preferred base addition salts include the sodium and ammonium salts.
xe2x80x9cEDTAxe2x80x9d means ethylenediaminetetraacetic acid and the base addition salts thereof, available from Aldrich Chemical Company, Milwaukee, Wis.
xe2x80x9cIVxe2x80x9d means intrinsic viscosity, which is RSV in the limit of infinite polymer dilution (i.e. the polymer concentration approaching zero). The IV is obtained by extrapolating the plot of RSV versus polymer concentration in the range of 0.015-0.045 weight percent polymer to the intercept of the y axis.
xe2x80x9c(Meth)acrylic acidxe2x80x9d means acrylic acid or methacrylic acid and the base addition salts thereof.
xe2x80x9cMonomerxe2x80x9d means a polymerizable allylic, vinylic or acrylic compound. The monomer may be anionic, cationic, nonionic or zwitterionic. Vinyl monomers are preferred, acrylic monomers are more preferred.
xe2x80x9cNonionic monomerxe2x80x9d means a monomer as defined herein which is electrically neutral. Representative nonionic monomers include acrylamide, methacrylamide, alkyl esters of acrylic and methacrylic acid such as methyl acrylate, acrylonitrile, methacrylonitrile, N-methyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, N-methylolacrylamide, N-vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, poly(ethylene glycol)(meth)acrylate, poly(ethylene glycol) monomethyl ether mono(meth)acryate, N-vinyl-2-pyrrolidone, glycerol mono((meth)acrylate), 2-hydroxyethyl(meth)acrylate, vinyl methylsulfone, vinyl acetate, and the like. Preferred nonionic monomers include acrylamide and methacrylamide. Acrylamide and methyl acrylate are more preferred.
xe2x80x9cReduced Specific Viscosityxe2x80x9d (RSV) is an indication of polymer chain length and average molecular weight. The RSV is measured at a given polymer concentration and temperature and calculated as follows:   RSV  =            [                        (                      η                          η              o                                )                -        1            ]        c  
wherein
xcex7=viscosity of polymer solution;
xcex7o=viscosity of solvent at the same temperature; and
c=concentration of polymer in solution.
The units of concentration xe2x80x9ccxe2x80x9d are (grams/100 mL or g/deciliter). Therefore, the units of RSV are dL/g. The RSV is measured at 30xc2x0 C. The viscosities xcex7 and xcex7o are measured using a Cannon-Ubbelohde semimicro dilution viscometer, size 75. The viscometer is mounted in a perfectly vertical position in a constant temperature bath adjusted to 30xc2x10.02xc2x0 C. The error inherent in the calculation of RSV is about + or xe2x88x922 dL/g. Similar RSVs measured for two linear polymers of identical or very similar composition is one indication that the polymers have similar molecular weights, provided that the polymer samples are treated identically and that the RSVs are measured under identical conditions.
In the case of the inverse emulsion polymers described herein, the inversion is conducted in 1% sodium hydroxide solution at an emulsion concentration of 1% by weight (based on the emulsion).
In the case of the water continuous emulsion polymers described herein, the polymer is hydrolyzed in 1% sodium hydroxide solution at an emulsion concentration of 1% by weight.
xe2x80x9cSalicylic acid containing monomerxe2x80x9d means a monomer unit having pendant salicylic acid group(s) as defined herein. Representative salicylic acid containing monomers include 3-acrylamidosalicylic acid and its base addition salts, 3-methacrylamidosalicylic acid and its base addition salts, 4-acrylamidosalicylic acid and its base addition salts, 4-methacrylamidosalicylic acid and its base addition salts, 5-acrylamidosalicylic acid and its base addition salts, 5-methacrylamidosalicylic acid and its base addition salts, 4-acrylamidosalicylic acid phenyl ester, 4-methacrylamidosalicylic acid phenyl ester, O-acetyl-4-acrylamidosalicylic acid, O-acetyl-4-methacrylamidosalicylic acid, 3-hydroxystyrene-4-carboxylic acid. 4-hydroxystyrene-3-carboxylic acid, and the like.
Preferred salicylic acid containing monomers are 4-methacrylamidosalicylic acid, 4-methacrylamidosalicylic acid phenyl ester, O-acetyl-4-methacrylamidosalicylic acid and O-acetyl-4-methacrylamidosalicylic acid phenyl ester.
xe2x80x9cSalicylic acid containing polymerxe2x80x9d means a water soluble or water insoluble polymer containing salicylic acid groups. The salicylic acid group may be incorporated into the polymer backbone or may be pendant to the polymer backbone. The salicylic acid containing polymers of this invention have a RSV of greater than about 1 dL/g when measured at 400 ppm (based on non-ionized acid mer units) in 2M NaNO3 as described herein. Preferred polymers have a RSV of greater than 14 dL/g. More preferred polymers have a RSV of greater than 20 dL/g. Polymers containing pendant salicylic acid groups are prepared by polymerizing one or more salicylic acid containing monomers with one or more nonionic or anionic monomers, or by grafting one or more salicylic acid groups onto a preformed natural or synthetic polymer backbone. Salicylic acid containing polymer comprising pendant salicylic acid groups preferably comprise from about 1 to about 90, more preferably from about 1 to about 20 and still more preferably from about 3 to about 10 mole percent of pendant salicylic acid groups.
Polymers incorporating salicylic acid into the backbone are prepared by condensation polymerization of one or more salicylic acid compounds and formaldehyde or by condensation polymerization of one or more salicylic acid acid compounds, formaldehyde and one or more compounds containing a reactive hydrogen center.
xe2x80x9cSalicylic acid groupxe2x80x9d means a group of formula 
where M is hydrogen, alkyl, aryl or a base addition salt; X is hydrogen or acyl; and the group 
represents an aryl group as defined herein, where 
is optionally substituted with xe2x80x94NO2, xe2x80x94OH, xe2x80x94SO3H. Representative salicylic acid groups include salicylic acid, salicylic acid methyl and phenyl ester, O-acetylsalicylic acid, O-acetylsalicylic acid methyl and phenyl ester, 2-hydroxy-5-nitrobenzoic acid, 2,3-dihydroxybenzoic acid, 2,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid, 5-sulfosalicylic acid, 1-hydroxy-2-napthoic acid, 3-hydroxyanthracene-2-carboxylic acid, 3- and 5-formylsalicylic acid, and the like. Preferred salicylic acid groups are salicylic acid, salicylic acid phenyl ester, O-acetylsalicylic acid and O-acetylsalicylic acid phenyl ester.
xe2x80x9cZwitterionic monomerxe2x80x9d means a polymerizable molecule containing cationic and anionic (charged) functionality in equal proportions, so that the molecule is net neutral overall.
Preferred Embodiments
In one preferred aspect of this invention, the salicylic acid containing polymer is selected from the group consisting of dispersion polymers, emulsion polymers, inverse emulsion polymers, dry polymers and solution polymers.
xe2x80x9cDispersionxe2x80x9d polymer means a water-soluble polymer dispersed in an aqueous continuous phase containing one or more inorganic salts. Representative examples of dispersion polymerization of water-soluble polymers in an aqueous continuous phase can be found in U.S. Pat. Nos. 4,929,655; 5,006,590; 5,597,859; and 5,597,858; and in European Patent Nos. 657,478; and 630,909.
Dispersion polymers are prepared by combining water, one or more inorganic salts, one or more water-soluble monomers, any polymerization additives such as chelants, pH buffers or chain transfer agents, and a water-soluble stabilizer polymer. This mixture is charged to a reactor equipped with a mixer, a thermocouple, a nitrogen purging tube, and a water condenser. The monomer solution is mixed vigorously, heated to the desired temperature, and then a water-soluble initiator is added. The solution is purged with nitrogen whilst maintaining temperature and mixing for several hours. During the course of the reaction, a discontinuous phase containing the water-soluble polymer is formed. After this time, the products are cooled to room temperature, and any post-polymerization additives are charged to the reactor. Water continuous dispersions of water-soluble polymers are free flowing liquids with product viscosities generally 100-10,000 cP, as measured at low shear rates. The advantages of preparing water-soluble polymers as water continuous dispersions are similar to those mentioned below in association with the inverse emulsion polymers. The water continuous dispersion polymers have the further advantages that they contain no hydrocarbon oil or surfactants, and require no surfactant for xe2x80x9cinversionxe2x80x9d or activation.
xe2x80x9cInverse emulsion polymerxe2x80x9d and xe2x80x9clatex polymerxe2x80x9d mean an invertible water-in-oil emulsion polymer consisting of an aqueous polymer phase dispersed as micron size particles in a hydrocarbon oil continuous phase, various emulsifying agents, and, potentially, an inverting surfactant. The advantages of polymerizing water-soluble monomers as inverse emulsions include 1) low fluid viscosity can be maintained throughout the polymerization, permitting effective mixing and heat removal, 2) the products can be pumped, stored, and used easily since the products remain liquids, and 3) the polymer xe2x80x9cactivesxe2x80x9d or xe2x80x9csolidsxe2x80x9d level can be increased dramatically over simple solution polymers, which, for the high molecular weight flocculants, are limited to lower actives because of viscosity considerations. The inverse emulsion polymers are then xe2x80x9cinvertedxe2x80x9d or activated for use by releasing the polymer from the particles using shear, dilution, and, generally, another surfactant, which may or may not be a component of the inverse emulsion.
Inverse emulsion polymers are prepared by dissolving the desired monomers in the aqueous phase, dissolving the emulsifying agent(s) in the oil phase, emulsifying the water phase in the oil phase to prepare a water-in-oil emulsion, in some cases, homogenizing the water-in-oil emulsion and polymerizing the monomers dissolved in the water phase of the water-in-oil emulsion to obtain the polymer as a water-in-oil emulsion. If so desired, a self-inverting surfactant can be added after the polymerization is complete in order to obtain the water-in-oil self-inverting emulsion.
The oil phase comprises any inert hydrophobic liquid. Preferred hydrophobic liquids include aliphatic and aromatic hydrocarbon liquids including benzene, xylene, toluene, paraffin oil, mineral spirits, kerosene, naphtha, and the like. Paraffin oil is preferred.
Free radical yielding initiators such as benzoyl peroxide, lauroyl peroxide, Vazo(copyright) 64, Vazo(copyright) 52, potassium persulfate and the like are useful in polymerizing vinyl and acrylic monomers. Vazo(copyright) 64 and Vazo(copyright) 52 are preferred. The initiator is utilized in amounts ranging between about 0.002 and about 0.2 percent by weight of the monomers, depending upon the solubility of the initiator.
Water-in-oil emulsifying agents useful for preparing the inverse emulsion polymers of this invention include sorbitan esters of fatty acids, ethoxylated sorbitan esters of fatty acids, and the like or mixtures thereof. Preferred emulsifying agents include sorbitan monooleate, polyoxyethylene sorbitan monostearate, and the like. Additional details on these agents may be found in McCutcheon""s Detergents and Emulsifiers, North American Edition, 1980. Any inverting surfactant or inverting surfactant mixture described in the prior art may be used. Representative inverting surfactants include ethoxylated nonylphenol, ethoxylated linear alcohols, and the like. Preferred inverting surfactants are ethoxylated linear alcohols.
The polymer is prepared by polymerizing the appropriate monomers at from about 1xc2x0 C. to about 85xc2x0 C. over about 1 to about 24 hours, preferably at a temperature of from about 40xc2x0 C. to about 70xc2x0 C. over about 3 to about 6 hours.
xe2x80x9cEmulsion polymerxe2x80x9d means a water-continuous dispersion of a water-insoluble polymer. The preparation of high molecular weight emulsion polymers such as poly(methyl acrylate) is described in U.S. Pat. No. 6,036,869. The polymer is rendered water soluble when activated with caustic solution to hydrolyze the ester groups and generate poly(sodium acrylate). Among the advantages of polymerizing in the water continuous format are that no hydrocarbon oil is present in the product (as is the case with the inverse emulsion systems), low viscosity fluids are obtained as products ( less than 100 cP is typical), and spills are easily cleaned up since the polymer is not water soluble until activated.
In the preparation of a water continuous dispersion, an aqueous mixture of one or more water soluble or water miscible surfactants is prepared such that a homogeneous solution results. Thereafter, one or more water insoluble monomers are added to this mixture with shear such that a water continuous emulsion is formed. After the emulsion has formed, the reaction vessel is cooled to below ambient temperature and purged with a nitrogen stream. After this, a stream of redox initiators are fed to the polymerization over time. Typical initiators include iron salts, peroxides and hydroperoxides, persulfates, bisulfites, and the like.
A typical polymerization may last three to four hours, after which time the emulsion is allowed to warm to ambient temperature, filtered and transferred to storage. Once the polymers are hydrolyzed in caustic solution, they may be characterized by the measurement of a RSV in a fashion similar to the inverse emulsion polymers.
xe2x80x9cDry polymerxe2x80x9d means a polymer prepared by drying a polymer prepared by xe2x80x9cgelxe2x80x9d polymerization. The preparation of high molecular weight water-soluble polymers as dry powders using a gel polymerization is generally performed as follows: an aqueous solution of water-soluble monomers, generally 20-60 percent concentration by weight, along with any polymerization or process additives such as chain transfer agents, chelants, pH buffers, or surfactants, is placed in an insulated reaction vessel equipped with a nitrogen purging tube. A polymerization initiator is added, the solution is purged with nitrogen, and the temperature of the reaction is allowed to rise uncontrolled. When the polymerized mass is cooled, the resultant gel is removed from the reactor, shredded, dried, and ground to the desired particle size.
Alternatively, dry polymers are prepared by spray drying emulsion, solution or dispersion polymers of this invention prepared as described herein.
Although it is not possible to prepare concentrated solutions of the same high molecular weight polymers prepared as inverse emulsions, water continuous dispersions, or gel polymers owing to the extremely high viscosities which are encountered, it is sometimes desirable to prepare lower molecular weight polymers of similar composition as solutions in water. To conduct a solution polymerization of water soluble monomers, the desired monomers are dissolved in water, generally at concentrations between 5 and 40%, along with any buffers, acid or caustic, chelants, chain transfer agents. The solution is purged with nitrogen and heated to the polymerization temperature. After the polymerization temperature is reached, one or more water soluble initiators is added. These initiators may be either of the azo type or of the redox type. Then, depending on the desired polymer characteristics, the temperature is either allowed to rise uncontrolled (adiabatic) or is controlled with cooling to remove the heat generated (isothermal). After the polymerization is complete, the solution of polymer can be removed from the reaction vessel, transferred to storage and characterized.
The polymer of this invention is also prepared by functionalizing natural or synthetic polymer with salicylic acid groups. For example, poly(acrylamide) containing pendant salicylic acid groups is prepared by Mannich reaction (formaldehyde, HCl) of poly(acrylamide). Similarly, naturally occurring polymers such as proteins can be functionalized with salicylic acid groups under Mannich conditions as described above. Alternatively, proteins and carbohydrates can be reacted with salicylic acid derivatives such as chloromethylated salicylic acid to incorporate pendant salicylic acid groups into the polymer.
Salicylic acid containing polymers of this invention that incorporate salicylic acid groups into the polymer backbone are prepared by condensation polymerization of substituted or unsubstituted salicylic acid and formaldehyde or by condensation polymerization of one or more salicylic acid compounds, formaldehyde and one or more compounds containing a reactive hydrogen center.
Representative compounds containing a reactive hydrogen center include ureas, amides such as acrylamide, amines such as melamine, dimethylamine, aniline, and the like, or aromatic compounds such as benzene, toluene, phenol, anisole, resorcinol, and the like. Preferred compounds that contain an active hydrogen center include urea, phenol, anisole, rescorcinol and melamine.
In a preferred aspect, the polymer is selected from the group consisting of dispersion polymers, emulsion polymers, inverse emulsion polymers, dry polymers and solution polymers.
In another preferred aspect, the polymer comprises pendant salicylic acid groups.
In another preferred aspect, the polymer is prepared by free radical polymerization of one or more salicylic acid containing monomers and one or more acrylate monomers selected from the group consisting of (meth)acrylic acid and alkyl esters of (meth)acrylic acid.
In another preferred aspect, the salicylic acid containing monomers are selected from the group consisting of 4-methacrylamidosalicylic acid, 4-methacrylamidosalicylic acid phenyl ester, O-acetyl-4-methacrylamidosalicylic acid and O-acetyl-4-methacrylamidosalicylic acid phenyl ester and the acrylate monomers are selected from the group consisting of methyl acrylate and acrylic acid.
In another preferred aspect, the polymer is selected from the group consisting of emulsion polymers and inverse emulsion polymers.
In another preferred aspect, the polymer comprises from about 80 to about 99 mole percent sodium or ammonium acrylate and from about 1 to about 20 mole percent 4-methacrylamidosalicylic acid, 4-methacrylamidosalicylic acid phenyl ester, O-acetyl-4-methacrylamidosalicylic acid or O-acetyl-4-methacrylamidosalicylic acid phenyl ester.
In another preferred aspect, the polymer comprises from about 88 to about 98 mole percent methyl acrylate, from about 1 to about 6 mole percent sodium acrylate and from about 1 to about 6 mole percent 4-methacrylamidosalicylic acid, 4-methacrylamidosalicylic acid phenyl ester, O-acetyl-4-methacrylamidosalicylic acid or O-acetyl-4-methacrylamidosalicylic acid phenyl ester.
In another preferred aspect, the polymer comprises backbone salicylic acid groups.
In another preferred aspect, the polymer is salicylic acid-formaldehyde copolymer.
In another preferred aspect, the polymer is a condensation polymer of one or more substituted or unsubstituted salicylic acid compounds, formaldehyde and one or more active hydrogen compounds.
In another preferred aspect, the salicylic acid compound is unsubstituted salicylic acid and the active hydrogen compound is selected from the group consisting of amides, ureas, amines and aromatic compounds.
Bayer process liquors generally are aqueous media containing dissolved sodium aluminate and red mud solids at various concentrations. Such liquors include the primary settler slurry or feed, which contains high concentration levels of both red mud and dissolved sodium aluminate, the red mud washing slurries, which have high red mud concentrations but lesser concentrations of sodium aluminate and total alkalinity, and the secondary clarification liquors, which are rich in dissolved sodium aluminate but contain much less red mud than the other types of liquors. Additional liquors include red mud slurries which are dewatered in centrifuges or on vacuum drum or disc filters, as well as red mud slurries which are flocculated to improve their mud stacking properties or to improve the tendency to release water from the mud slurry. As discussed above, the separation of the red mud from the sodium aluminate and its aqueous phase is continued from the primary settlement stage until concentrated red mud is eliminated from the process circuit, and from the primary settlement stage until the clarified liquor is subjected to the alumina trihydrate crystallization.
The flocculation of red mud, which routinely precedes or follows either settling or filtration, is most difficult in the primary settlement stage because of the high concentration of fine particles, and the high concentration of total alkalinity. Improvement of the flocculation effectiveness in the primary settlement stage is extremely important to the entire Bayer process. By reducing the level of suspended solids that remain in the supernatant above the settled mud solids formed in the primary settler liquor, the solids to be removed during secondary clarification stages are reduced.
In the Bayer process, the bauxite ore is digested under highly alkaline conditions, and the typical primary settler liquors are routinely highly alkaline, containing sodium hydroxide, sodium aluminate, and commonly sodium carbonate. The total alkalinity of the primary settler feed, that is the liquor charged to the primary settlement stage, is typically from about 100 to 300 grams per liter of settler feed, as sodium carbonate equivalent. The solids contents of typical primary settler feeds vary from about 25 to about 85 grams per liter of settler feed.
Primary settler feed means the Bayer process digested slurry as charged to the flash tanks or other vessels emptying into the primary settler. Such feed may be an admixture of the digested slurry plus dilution liquor, and the dilution liquor is routinely the counter current technique wash water from the red mud washing stages discussed above. The primary settler feed differs from the liquors or slurries subjected to clarification and/or separation in the secondary clarification stage or the red mud washing stages by composition as to the solids content, dissolved sodium aluminate content, and total alkalinity. The primary settler feed also differs from the liquors or slurries in that no insoluble fraction thereof has received an earlier flocculation treatment.
Accordingly, improved clarification of Bayer process primary settler liquors is one aspect of this invention. Nonetheless in its broadest sense, this invention is directed to the clarification and settling of red mud-containing liquors in any aspect of mineral processing wherein such red mud is found. For example, the polymers of this invention may be used in the counter-current wash liquors, primary settler liquors of the Bayer process, as well as in red mud which is dewatered in centrifuges or by vacuum filtration (drum filters, and disc filters among others) or in the settler overflow to improve filtration in the polishing filters (either pressure or sand filters) or in red mud which is flocculated after the last mud washing stage to improve the mud stacking properties in the mud disposal area, or in red mud which must be made useful for other purposes.
Upon flocculation of a primary settler feed, using the polymers of this invention, a liquor/mud interface will form upon settling of the mud solids. The supernatant liquor is low in suspended solids (generally ranging from about 10 to about 500 mg/l) and overlies a mud layer. The lower mud layer contains the flocculated material, and as discussed above, is comprised of both red mud solids (generally ranging from about 10 to about 70% mud solids by weight) and some amount of pregnant liquor. The overlying supernatant is the liquor that is separated for secondary clarification, again as discussed above. The interface between the supernatant mud liquor and the mud layer is clearly seen in some cases, but the supernatant is not entirely free of suspended solids, appearing instead as a translucent liquid. The present invention diminishes the amount of suspended solids in such supernatant, and hence decreases the extent of secondary clarification required to obtain a given purity of sodium aluminate solution. Use of the polymers of this invention also reduces or eliminates the need for starch by improved supernatant liquor clarity and improved red mud stability, and the rheological properties of the concentrated red mud slurry.
The digested slurry is typically discharged from the flash tanks at elevated temperatures. The primary settler feed is generally not further cooled before charging to the primary settlement stage other than the cooling which may occur when a digested slurry is optionally admixed with the liquor from the first red mud wash stage to form a primary settler feed. The flocculation of the primary settler feed is conducted at atmospheric pressures and at elevated temperatures of from about 80xc2x0 C. to about 110xc2x0 C. The flocculation of the primary settled feed can also be conducted at elevated pressures and temperatures as high as 200xc2x0 C.
The following applies to any aspect of this invention. High and/or lower molecular weight salicylic acid containing polymers may be used in combination with any conventional nonionic polysaccharide flocculant such as starch, dextran, alginate and flour, and anionic flocculants such as homopolymers of acrylic acid or acrylates, co-ploymers of acrylic acid or acrylates containing at least 50 molar percent acrylic acid or acrylate monomers, alkali metal, alkaline earth metal or ammonium salts of said acids, or a polyacrylate alkyl ester of acrylate copolymer with 60 to about 90 percent of the alkyl ester groups hydrolyzed. Any of the above anionic flocculants may be further functionalized with pendant hydroxamic acid groups. The salicylic acid containing polymer may be added before, after or simultaneously with any of the foregoing.
The polymers may also be utilized to treat the last stage washer underflow at the mud disposal site to improve mud stacking, or more rapid release of liquor from the mud. Moreover, the polymers may also be utilized for the treatment of mud filters, including but not limited to drum and vacuum filters.
Water soluble polymers of this invention are used as follows: A solution of the polymer is prepared in an appropriate dilution water stream typically as an about 0.1 to about 1 weight percent polymer active solution. This solution is added to the digested bauxite sodium aluminate process stream containing suspended solids in an amount sufficient to settle said solids. For example, the polymer is injected into the feed-line upstream of the settling vessel and/or added to the center-well of the settling vessel.
Alternatively, water-continuous polymers of this invention are added neat or as a dilute solution to the primary liquor feed of a Bayer process. The water continuous polymers hydrolyze in situ in the Bayer process liquor itself. In more detail, co-polymers and or terpolymers formed from acrylic acid and/or acrylic acid esters and salicylic esters may not be active as red mud flocculants until the ester groups are hydrolyzed. When placed in the Bayer process liquor in the presence of red mud, the high alkalinity and the high temperatures convert the polymers to effective red mud flocculants by hydrolyzing the various ester groups to ionized acrylic acid and salicylic acid groups. Furthermore, the polymer does not hydrolyze instantly, but rather over time. Therefore, the poly (acrylic acid/acrylic acid ester/salicylic acid ester) essentially is being activated continuously.
The salicylic acid containing polymers are injected upstream from the primary settler, such as in one of the flash tanks or between the flash tanks and the primary settler feed well where there is sufficient temperature and residence time to allow the hydrolysis of the polymer. The hydrolysis will progress as the polymer and mud make their way from the flash tanks down the various piping and into the primary settler.
The red mud containing liquor may be a primary settler feed, a mud washer feed, a centrifuge feed or the polishing filter feed (pressure or sand filter). The feed may be from a digester blow-off, diluted digester blow-off, primary settler underflow, washer underflow, or a combination of settler and washer underflows with other process streams including but not limited to settler overflow, washer overflows, lake return water or raw water. The polymers described herein, when utilized to treat Bayer process red-mud containing liquor, result in an increase in both clarity and settling rate.
The water-continuous polymers of this invention may be hydrolyzed in a caustic solution, using various plant liquor streams alone or combinations thereof such as spent liquor, pregnant liquor, any washing circuit overflow liquor containing some caustic, lake return water, and/or condensate waters with addition of caustic, prior to being added to the primary liquor feed of a Bayer process as details above.
In another preferred aspect, the polymer is hydrolyzed prior to addition to the Bayer process liquor.
In another preferred aspect, the Bayer process liquor is selected from settler feed, settler overflow, digestion blow-off, mud washer in the washer train, feed to the primary polishing filters, feed to a mud settler, feed to the primary alumina crystallization tanks, feed to the secondary and tertiary alumina classifers or trays, feed to hydrate filters or feed to a centrifuge.
In another preferred aspect, one or more anionic or nonionic flocculant(s) are added to the liquor.
In another preferred aspect, the nonionic flocculant is starch, dextran or flour.
In another preferred aspect, the anionic flocculant comprises poly (meth)acrylic acid.
In another preferred aspect, the poly (meth)acrylic acid is selected from the group consisting of poly(meth)acrylic acid, poly(meth)acrylic acid containing pendant hydroxamic acid groups, poly (alkyl (meth)acrylate), (meth)acrylic acid/alkyl (meth)acrylate copolymers, (meth)acrylic acid/acrylamide copolymers, (meth)acrylic acid/acrylamide copolymers containing pendant hydroxamic acid groups, (meth)acrylic acid/acrylamide/alkyl (meth)acrylate terpolymers, and (meth)acrylic acid/acrylamide/AMPS terpolymers.
In another preferred aspect, the anionic flocculant is added prior to the salicylic acid containing polymer.
In another preferred aspect, the anionic flocculant is added after the salicylic acid containing polymer.
In another preferred aspect, the anionic flocculant is added together with the salicylic acid containing polymer.
In another preferred aspect, the salicylic acid containing polymer has an RSV of from about 14 to about 21 dL/g and the anionic flocculant has an RSV greater than about 31 dL/g.